The hottest shrink packaging and stretch packaging

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Heat shrinkable packaging and stretch packaging technology (Part 1)

heat shrinkable packaging is a packaging method that uses heat shrinkable plastic film to wrap products and packages, and then heats the film to shrink and wrap products or packages tightly; Stretch packaging is a method of wrapping products or packages with stretchable plastic film under normal temperature and tension. These two packaging methods have different principles, but the packaging effects are basically the same. They are similar to the packaging technology, but the materials and working principles are completely different

heat shrinkable packaging technology

heat shrinkable packaging technology and heat forming and body fitted packaging technology entered China in the 1970s, and have been rapidly developed and popularized. Therefore, they are considered to be one of the three fastest-growing packaging technologies in this century, and also a promising packaging technology

I. commonly used shrink film

shrink film is the most important kind of shrink packaging materials. According to the characteristics of thermoplastics that will recover under heating conditions, in the process of making films from plastic raw materials, heat stretching is carried out in advance, and the shrink films are made after cooling

the manufacturing methods of shrinkable films are divided into sheet and tube. Flake film is made into flakes, and then stretched along the longitudinal and transverse axes of the film, which is called secondary stretching method, or stretching in two directions at the same time, which is called primary stretching method; The cylindrical film is first made into a cylindrical shape, and then stretched twice or once

1. Main performance indicators of shrink film

(1) shrinkage and shrinkage ratio

shrinkage includes longitudinal and transverse. The test method is to measure the film length L1 first, and then immerse the film at 120o С 1 ~ 2S in glycerol, take it out and cool it with cold water, then measure the length L2, and calculate it according to the following formula:

l1 - L2

shrinkage (%) =----- × 100%

L enter the system setting interface 1

where L1 is the length of the film before shrinkage; L2 is the length of the film after shrinkage

at present, the shrinkage rate of shrink film used for packaging is generally equal in both vertical and horizontal directions, about 50%; However, in special cases, there are also unidirectional contractions, with a shrinkage rate of about 25% - 50%. There are also partially extended films with unequal shrinkage in both vertical and horizontal directions

the ratio of contraction ratio in both vertical and horizontal directions is called contraction ratio

(2) shrinkage tension

shrinkage tension refers to the tension applied to the package after the film shrinks. The size of shrinkage tension under shrinkage temperature is closely related to the protection of products. Rigid products such as packaging metal cans allow large shrinkage tension, while some fragile or wrinkle prone products will be deformed or even damaged if the shrinkage tension is too large. Therefore, the shrinkage tension of the shrink film must be selected appropriately

(3) shrinkage temperature

the shrinkage film starts to shrink when it reaches a certain temperature after heating, and stops shrinking when the temperature rises to a certain height. The temperature within this range is called the shrinkage temperature. For the packaging operation, the temperature reached when the package is heated in the shrinkage channel of the enterprise without recovery equipment, and the film shrinks to produce a predetermined tension is called the shrinkage temperature under this tension. There is a certain relationship between shrinkage temperature and shrinkage, and various films are different. The figure shows the temperature shrinkage curve of PVC, polyethylene and polypropylene. In shrink packaging, the lower the shrinkage temperature, the smaller the adverse effect on the packaged products, especially fresh vegetables, fruits and textiles

(4) heat sealing

in shrink packaging, before heating shrinkage, it is necessary to heat seal two or three sides first, and the sealing seam is required to have high strength

2. Properties and uses of commonly used shrink films

at present, the most commonly used shrink films are PVC, polyethylene and polypropylene, polyvinylidene chloride, polyester, polystyrene, ethylene vinyl acetate copolymer and chlorinated rubber

(1) PVC shrinkage film

shrinkage temperature is relatively low and has a wide range, as shown in the figure. The shrinkage temperature is 40 ~ 160 ℃, and the heating channel temperature is 100 ~ 160 ℃. Fast thermal shrinkage and good performance. The package is transparent and beautiful after processing, and the heat sealing part is also very neat. The oxygen permeability is lower than that of polyethylene, and the moisture permeability is high, so it is more suitable for the packaging of vegetables and fruits with more moisture. Its disadvantage is that it has low impact strength and is easy to become brittle at low temperature, so it is not suitable for transportation and packaging. In addition, the sealing strength is poor, which will decompose and produce odor during heat sealing. When the plasticizer in it changes, the film is easy to break and lose gloss. At present, PVC film is mainly used for packaging groceries, food, toys, fruits and textiles

(2) polyethylene

is characterized by high impact strength, low price, firm sealing, and is mostly used for transportation and packaging. The gloss and transparency of polyethylene are worse than that of PVC. In operation, the shrinkage temperature is about 20 ~ 30 ℃ higher than that of PVC. Therefore, blast cooling device shall be installed in the rear section of the heat shrinking channel

(3) polypropylene

has the main advantages of good transparency and gloss, the same as cellophane, good oil resistance and moisture resistance, and strong shrinkage tension. L1 after displacement are: ab+2bc or ab+bc+bc1 point is poor heat sealing, low sealing strength, high shrinkage temperature and narrow range. Typical uses are multiple packages of audio tapes and records

(4) other films

polystyrene is mainly used for letter packaging, and polyvinylidene chloride is mainly used for meat packaging

the recently appeared ethylene vinyl acetate copolymer shrink film has high impact strength, high transparency, low softening point, high melting temperature, good heat sealing performance, low shrinkage tension, and the packaged products are not easy to be damaged. It is suitable for the packaging of items with protrusions or special-shaped items. It is expected to have a great development in the future

II. Methods of shrink packaging

shrink packaging has two methods: manual heat shrinking and mechanical heat shrinking packaging. Manual heat shrinking usually involves wrapping the packaged items manually, and then blowing hot air on the packaged items with tools such as hot air gun to complete heat shrinking packaging. This method is simple and rapid. It is mainly used for heat shrinkable packaging of packages that are not suitable for mechanical packaging, such as products packaged on large pallets or single shaped products with large volume. The thermal power is 1.5 × 108j/h spray gun can pack a 2M2 product, and the heat shrinkage only takes about 2min. In addition to the spray gun, only one liquefied gas tank is needed. This method is convenient and economical, and it is worth popularizing in China

the mechanical heat shrinkable packaging process is generally carried out in two steps. First, the products and packages are pre wrapped mechanically, that is, the products are packaged with shrink film, and the necessary openings and seams are heat sealed. Then, the heat shrinkable, and the pre wrapped products are put into the heat shrinkable equipment for heating. This method is the most commonly used, so it is also the content we focus on

in heat shrinkable packaging, the packaging film used for wrapping articles with heat shrinkable film includes cylindrical film, flat film and folded film. The folded film can be cut from cylindrical film or folded from flat film. Wrapping articles with cylindrical film has fewer bag making and sealing joints, but the mechanical wrapping efficiency is low and the structure is complex

1. pre wrapping operation

when pre packaging, the film size should be 10% - 20% larger than the commodity size. If the size is too small, the filling is inconvenient, and the shrinkage tension is too large, which may tear the film; If the size is too large, the shrinkage tension is not enough, and the package is not tight or flat. The thickness of the shrink film used can be determined according to the size, weight and the required shrink tension of the commodity

the commonly used heat shrinkable packaging methods are as follows:

(1) open at both ends: when the cylindrical membrane is used, the opening of the cylindrical membrane needs to be expanded, and then the articles are sent into the cylindrical membrane with the help of the chute. The size of the cylindrical membrane is about 10% larger than the size of the "article", as shown in the figure. This method is more suitable for wrapping cylindrical objects, such as batteries, paper rolls, wine bottles, etc. The advantage of using cylindrical film packaging is that it reduces 1 ~ 2 sealing seams and has a beautiful appearance. The disadvantage is that it can not adapt to the diversification of products. It is usually used for the packaging of single variety and large quantities of products

there are two ways to wrap articles with flat film: single flat film and double flat film. The film should be wider than the object. When wrapping the object with a single flat film, first unfold the flat film, and then feed the wrapped object towards the middle of the flat film to form a horseshoe shaped wrapping, and then fold it into a closed sleeve, which is sealed by hot melting. Double flat films are used for wrapping, that is, the upper and lower films are used. After the previous package is sealed and cut, the two films are sealed, and then the product is pushed to the vertical film by machine or manually. After it is in place, the sealing and cutting mechanism falls down, and the other side of the product is sealed and cut at the same time. After thermal shrinkage, both ends of the package shrink to form an oval opening. The operation process is shown in the figure, Flat film packaging is not limited by the variety of products. Flat film is mostly used for the packaging of single or multiple products with square shape, such as multiple boxed products. (to be continued)

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